If you speak to Pikkupstix in Taverley he will give you an option to enchant the helmet allowing you to store Summoning scrolls within the helmet. This will turn the helmet into a Full slayer helmet (e).
Using any Summoning scrolls on the enchanted helmet will store them inside it, giving a number of charges equal to the number of scrolls stored. Doing so will turn the helmet into a Full slayer helmet (c).
For 500 Slayer reward points, you can learn how to add rings to your slayer helmet. This will allow you to add Rings of slaying or Ferocious rings to the helmet granting the ring's teleport benefits. This will change the helmet into a Full slayer helmet (f). These helmets can also be enchanted and charged with Summoning scrolls as described above, giving either a full slayer helmet (ef) or a full slayer helmet (cf) respectively.

You can spend 100 Slayer reward points to upgrade your slayer helm into a Reinforced slayer helmet. To do this you must be wearing the helmet when buying the upgrade. If you lose or destroy the upgraded helmet you will need to spend your slayer reward points again.